The use of paper under conditions in which the paper must be used in the wet state or to absorb moisture has become one of the major utilities of paper products. Paper normally achieves its integrity or fiber to fiber strength by bonding between the fibers, possibly hydrogen bonding of the various molecular components of the fiber. Such fiber to fiber bonding provides adequate strength for papers used in the dry condition, such as writing or printing papers or packaging which is not subjected to outdoor atmospheric conditions. However, with the advent of disposable towels prepared from paper, disposable paper diapers and the need for packaging which maintains its strength under moisture conditions such as outdoor or atmospheric applications, it has become necessary to provide additional bonding for these paper products in order to maintain what is called wet-strength. Wet-strength is developed in paper products by various coating methods which provide continuous or discrete depositions of polymeric resins which effectively bond portions of the paper product. Such wet-strength resin coated paper products are necessary for manufacture of disposable paper kitchen towels and disposable baby diapers. Other uses of wet-strength papers are readily known in the prior art.
Prior to the advent of wet-strength resin coated paper products, recycled paper, waste paper, clippings, and paper broke were easily recycled to the paper forming processes of paper mills by various mild digestions of the recycled paper before being returned to the paper furnish. Even the use of water soluble resins for paper coating provided no problem for those interested in recycling the various paper wastes.
However, the wet-strength resins have provided difficult problems for the reuse of waste paper, such that such waste paper in many instances is burned or landfilled rather than subject the paper to expensive or poor yield cleansing processes.
A process for recycling printed paper to a useable pulp is described in U.S. Pat. No. 1,245,839. The invention is specifically concerned with the removal of ink and other printed coloring matter from paper. The paper is subjected to an aqueous soap solution which saponifies the oil or fatty portions of the ink. Air is used to separate the ink and carbon pigment from the paper by froth flotation. Ozone or other gases can be used to bleach the pulp. However this patent is not concerned with the removal of polymer or resin coatings from paper.
In U.S. Pat. No. 2,042,465, the problem of de-inking uncoated waste paper is again addressed. A caustic soda or soapy solution is used to slurry the paper to a pulp, while air is passed up through the pulp to oxygenate the oil phase of the ink. The oxidized ink is then separated from the pulp. Again, this reference does not concern removal of wet-strength resins.
In U.S. Pat. No. 2,872,313 a process for defibering wet-strength paper broke is set forth in which oxidizing salts in a dilute aqueous solution are utilized to loosen the fiber to resin bonds. The paper broke is soaked in the oxidizing solution with preferably mild agitation. The oxidizing salt agents can be selected from sodium hypochlorite, sodium chlorite, sodium chlorate, potassium chlorate, ammonium persulfate and sodium peroxide. The solution utilized generally has a pH between 7 and 10. Elevated temperatures may be utilized in the range of 115.degree. to 180.degree. F.
The removal of thermosetting wet-strength resins from paper is addressed in U.S. Pat. No. 3,245,868. Inclined screw fed vessels are used to handle the pulp. The resin can be removed by either acidic solutions for formaldehyde resins or basic solutions (caustic soda) for removing polyamide resins. Temperatures as high as 149.degree. C. are contemplated, but the patent does not direct itself to paper brightness enhancement. Additionally, the patent contemplates caustic strengths to dry fiber pulp of only 4%.
U.S. Pat. No. 3,427,217 discloses a recycle process for wet-strength resin coated broke wherein an oxidizing salt such as sodium hypochlorite is used to facilitate the pulping of the coated paper. Sodium hydroxide can be used to provide a basic solution.
Additional references which are directed to broke recovery are: U.S. Pat. Nos. 3,407,113, 3,425,897, 3,438,851; and 3,933,578.